Various techniques have heretofore been proposed for pipe inspection, including magnetic inspection by leakage field and/or eddy current techniques, gamma or X-ray techniques, ultrasonic techniques, television techniques and photographic techniques. Each of such techniques may be used to advantage in certain applications. However, each technique, as heretofore employed, has serious limitations in other applications, such as, for example, in the inspection of pipeline used for transport of natural gas from offshore stations. Natural gas cannot be safely treated offshore and in a raw state it may include the combination of carbon dioxide and free water which produces carbonic acid and causes corrosion of steel, especially when the temperature is relatively high as is often the case. The corrosion is augmented by an erosion effect where the rate of flow is high and at bends and over weld beads or the like where the flow is turbulent.
If localized damage to the pipe could be detected before it presents a serious problem, it would be possible to effect repair or to otherwise take corrective measures. For example, it is possible to move devices known as "pigs" through a pipe and by moving a series of such pigs through a pipe, it can be cleaned, etched, washed, dehydrated and then coated with a protective coating such as an epoxy. Such a procedure is, of course, relatively expensive especially in that the pipeline cannot be used for an extended period of time, and it is not desirable to utilize the procedure unless and until the necessity therefor can be established by a suitable inspection. Also, even after a protective coating is applied, there is the possibility of breaks in the coating and further periodic inspection is desirable to make certain that the coating is properly protecting the pipe. Accordingly, a reliable and accurate way of inspecting such pipeline for defects would be highly desirable.
As above indicated, prior art techniques are not satisfactory. Magnetic inspection, if properly performed, has many advantages including a high degree of sensitivity to flaws of types which may cause problems and the ability to cover large distances with suitable magnetic recording or other information storage equipment. However, there are disadvantages including the necessity of having operators with a high degree of skill in order to obtain an accurate interpretation of the information obtained. Ultrasonic techniques have a serious disadvantage in that it is difficult to couple the ultrasonic energy into the wall of a pipe and television techniques have a serious disadvantage, at least in the present state of development, in that a high sensitivity and high resolution is difficult to obtain especially if a substantial length of pipe is to be inspected and the information is to be recorded. If the information is to be transmitted through a cable, there is a problem as to loss of sensitivity at great distances and the difficulty in moving a cable having a large mass through a pipeline. Photographic techniques as heretofore proposed have had limitations not recognized in the prior art particularly with respect to obtaining records which can be easily and accurately interpreted and also with respect to reliability.
With regard to specific prior art disclosures, the Nettles et al. U.S. Pat. No. 2,892,150 discloses a device movable through a pipe and including seal or cup members for sealing engagement with the inside of the pipe to permit drive of the device by fluid pressure. A magnetic testing device is provided for detecting variations in the thickness of the wall of the pipe and indications obtained are recorded on a recording tape. To correlate the recorded indications with the position of the device while permitting variations in speed, the recording tape is driven from a wheel engaged with the inside surface of the pipe.
The Green et al. U.S. Pat. No. 3,064,127 discloses a pipeline survey instrument in which a survey capsule or "pig" carries cups or sealing flanges engageable with the inside of a pipe so that the capsule may be moved through the pipe by fluid pressure. The capsule carries various instruments for making tests including a radiation analysis assembly for detecting cavities in the pipe wall, a caliper assembly for measuring the inside diameter of the pipe, a water detector and an electric current sensing assembly. In addition, a recording assembly is provided for recording the results of the various measurements. Since the pig may move at various speeds, the recording assembly is driven from a wheel engaged with the inside of the pipe and the measurements are thereby correlated with the distance along the pipe regardless of the speed of movement of the pig.
No camera is carried by the pig of the Green et al. patent. However, the use of a camera for the internal inspection of pipe has been proposed in other prior art references. For example, the Pulfer U.S. Pat. No. 3,244,085 discloses a capsule movable along the inside of a pipe or tube and including a camera and illumination means. Film exposures are taken at spaced locations along the length of the pipe or tube, the direction of each exposure being controlled by a shutter or by control of the duration of the operation of the illumination means. Another similar disclosure is contained in the Watts et al. U.S. Pat. No. 3,667,359 in which the camera and illumination means are supported by a pig which carries cups or sealing flanges engageable with the inside of the pipe so as to be movable by fluid pressure, as in the Green et al. patent. The Watts et al. assembly also includes a wheel engageable with the inside of the pipe and connected to the camera so as to correlate the pictures with the distance along the pipe.
In both the Pulfer and the Watts et al. devices, the pictures are taken from one end of the capsule or pig, the viewing axis of the camera being coincident with the axis of the pipe and the illumination means being positioned radially outside the axis of the camera.
The Watts et al. U.S. Pat. No. 3,667,359 also discloses an arrangement for directing a gas spray across the surface of a lens assembly for preventing fogging thereof.